ILOVEYOU, sometimes referred to as Love Letter, was a computer worm that attacked tens of millions of Windows personal computers on and after 5 May 2000[1] local time in the Philippines when it started spreading as an email message with the subject line "ILOVEYOU" and the attachment "LOVE-LETTER-FOR-YOU.txt.vbs". The latter file extension (in this case, 'VBS' - a type of interpreted file) was most often hidden by default on Windows computers of the time, leading unwitting users to think it was a normal text file. Opening the attachment activated the Visual Basic script. The worm did damage on the local machine, overwriting image files, and sent a copy of itself to all addresses[2] in the Windows Address Book used by Microsoft Outlook. In contrast, the Melissa only sent copies to the first 50 contacts.

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On the machine system level, ILOVEYOU relied on the scripting engine system setting (which runs scripting language files such as .vbs files) being enabled, and took advantage of a Microsoft algorithm for hiding file extensions. Windows hid file extensions by default; to do this, it parsed file names from right to left, stopping at the first period character. The attachment, which had two periods, could thus display the inner fake "txt" file extension. Text files are considered to be innocuous, as they are normally incapable of running executable code. The worm also used social engineering to entice users to open the attachment (out of actual desire to connect or simple curiosity) to ensure continued propagation. Systemic weaknesses in the design of Microsoft Outlook and Microsoft Windows were exploited that allowed malicious code capable of complete access to the operating system, secondary storage, and system and user data simply by unwitting users clicking on an icon.

Messages generated in the Philippines began to spread westwards through corporate email systems. Because the worm used mailing lists as its source of targets, the messages often appeared to come from acquaintances and were therefore often regarded as "safe" by their victims, providing further incentive to open them. Only a few users at each site had to access the attachment to generate millions more messages that crippled mail systems and overwrote millions of files on computers in each successive network.

The virus originated in the Pandacan neighborhood of Manila in the Philippines on May 5, 2000, thereafter following daybreak westward across the world, moving first to Hong Kong, then to Europe, and finally the United States, as employees began their workday that Friday morning.[3][4] The outbreak was later estimated to have caused US $5.5-8.7 billion in damages worldwide,[5][6] and estimated to cost the US $15 billion to remove the worm.[7] Within ten days, over fifty million infections had been reported,[8] and it is estimated that 10% of internet-connected computers in the world had been affected.[6] Damage cited was mostly the time and effort spent getting rid of the infection and recovering files from backups. To protect themselves, The Pentagon, CIA, the British Parliament and most large corporations decided to completely shut down their mail systems.[9] This virus affected over 45 million computers and was one of the world's most dangerous computer related disasters.

The worm then searched connected drives and replaced files with extensions JPG, JPEG, VBS, VBE, JS, JSE, CSS, WSH, SCT, DOC, HTA, MP2, and MP3 with copies of itself, while appending the additional file extension VBS, making the user's computer unbootable. However, the MP3 and sound related files are hidden and not overwritten.

The worm propagated itself by sending out one copy of the payload to each entry in the Microsoft Outlook address book (Windows Address Book). It also downloaded the Barok trojan renamed for the occasion as "WIN-BUGSFIX.EXE".

The fact that the virus was written in VBS provided users a way to modify the virus. A user could easily modify the virus to replace important files in the system, and destroy it. This allowed many variations of ILOVEYOU to spread across the internet, each one doing different kinds of damage.[10]

After surveillance and investigation by Darwin Bawasanta of Sky Internet, the NBI traced a frequently appearing telephone number to Ramones' apartment in Manila. His residence was searched and Ramones was arrested and placed on inquest investigation before the Department of Justice (DOJ). Onel de Guzman was likewise arrested in absentia.

At that point, the NBI were unsure what felony or crime would apply.[12] It was suggested they be charged with violating Republic Act 8484 (the Access Device Regulation Act), a law designed mainly to penalise credit card fraud, since both used pre-paid (if not stolen) Internet cards to purchase access to ISPs. Another idea was that they be charged with malicious mischief, a felony (under the Philippines Revised Penal Code of 1932) involving damage to property. The drawback here was that one of its elements, aside from damage to property, was intent to damage, and de Guzman had claimed during custodial investigations that he may have unwittingly released the worm.[13]

To show intent, the NBI investigated AMA Computer College, where de Guzman had dropped out at the very end of his final year.[12] They found that, for his undergraduate thesis, de Guzman had proposed the implementation of a trojan to steal Internet login passwords.[14] This way, he proposed, users would finally be able to afford an Internet connection. The proposal was rejected by the College of Computer Studies board,[13] prompting de Guzman to cancel his studies the day before graduation.

Since there were no laws in the Philippines against writing malware at the time, both Ramones and de Guzman were released with all charges dropped by state prosecutors.[15] To address this legislative deficiency,[12] the Philippine Congress enacted Republic Act No. 8792,[16] otherwise known as the E-Commerce Law, in July 2000, just two months after the worm outbreak. In 2002, the ILOVEYOU virus obtained a world record for being the most virulent computer virus at the time.[17]